The number of new human cases of Lyme borreliosis in the United States has recently leaped to record highs, suggesting that the populations of the disease-causing genotypes of Borrelia burgdorferi sensu stricto (s.s.) are expanding at an accelerating rate. The broad and long-term objectives of this application are to disentangle the evolutionary processes acting on the bacteria to facilitate modelling the spread of disease-causing genotypes of B. burgdorferi s.s. in the United States, and to provide rationales for developing new diagnostic tools and for designing specific control methods of Lyme borreliosis. There is urgent need to establish a Multilocus Sequence Typing (MLST) scheme for B. burgdorferi s.s. based on housekeeping genes in order to analyze the recent population growth, range expansion and population dynamics of pathogenic genotypes in the United States. The specific aims are (i) to establish a MLST scheme of B. burgdorferi s.s. based on housekeeping genes, (ii) to assess the present day genetic population structures of wild-type B. burgdorferi s.s. populations in the Northeast United States using MLST and landscape genetics, and (iii) to infer the recent demographic patterns of B. burgdorferi s.s., such as population bottlenecks and expansions. Isolates of B. burgdorferi s.s. derived from questing ticks will be sampled at different geographical scales across New England and will be genotyped by MLST. Using a new algorithm (eBURST), the patterns of evolutionary descent amongst the bacterial populations will be clarified. The spatial structure of the Borrelia populations will be determined by spatial statistics and related to landscape variables. Using GIS, a synthesis map will be generated that displays an interpolated genetic surface. The relative importance of the evolutionary processes that have been shaping the bacterial populations, such as mutation, recombination, migration and selection, will be estimated. The analyses of wild-type Borrelia populations by MLST at housekeeping genes and by landscape genetics is novel and explorative, and could break new ground in our understanding of the evolution of emerging vector-borne zoonotic pathogens. [unreadable] [unreadable] [unreadable]